Optical measuring device for test strips

ABSTRACT

An optical measuring device ( 10 ) for detecting the coloring of test fields of a test strip ( 12 ), which is to be wetted with a liquid for detecting substances in said liquid, whereupon the reflectivity of the test fields changes depending on the concentrations of the substances to be detected, comprising: a measuring plane ( 14 ) wherein said test strip ( 12 ) is to be placed; an illumination device ( 16 ) for illuminating the measuring plane ( 14 ); a planar image sensor ( 36 ); an optical system for imaging said measuring plane ( 14 ) onto the image sensor ( 36 ); and an electronic evaluation unit ( 52 ) for evaluation of the signals detected by said image sensor ( 36 ), said illumination device ( 16 ) comprises light sources ( 18, 20, 22 ) or other means of different colors which serve to alternatively illuminate said measuring plane ( 14 ) in different colors, wherein said electronic evaluation unit ( 52 ) detects the coloring of the test fields from the images obtained under different color illuminations.

The present invention concerns an optical measuring device for detectingthe coloring of test fields of a test strip, which is wetted with aliquid for detecting substances in said liquid, whereupon thereflectivity of the test fields changes depending on the concentrationof the substances to be detected, comprising

-   -   a measuring plane wherein said test strip is to be placed,    -   an illumination device for illuminating the measuring plane,    -   a planar image sensor,    -   an optical system for imaging said measuring plane onto the        image sensor, and    -   an electronic evaluation unit for evaluation of the signals        detected by said image sensor.

Optical measuring devices of this kind are e.g. known as part of teststrip analysis apparatus, which are used for examining urine or bloodtest strips in doctor's practices, hospitals or medical laboratories.The test strips to be analyzed often comprise a plurality of testfields, each of which serves to detect another substance. In suchmeasuring devices, the test strip is illuminated as a whole and imagedonto the image sensor. From the image obtained, the electronicevaluation unit may then determine the coloring of all test fieldsacross the test strip. Thus, it is not necessary to scan the individualtest fields and the measurement of the test strip may be conducted in ashorter time, wherein the structure of the measuring device is simpler.

For detecting the coloring of a test field, its reflectivity for lightof three different wavelengths has to be determined. In a conventionalmeasuring device, the test strip is illuminated with white light andimaged onto an image sensor comprising sensor elements, each of which issensitive to one of three wavelengths (colors). The light-sensitiveelements are arranged in a planar matrix whose lines each comprisesensor elements sensitive to the light of the same color. Thus, thereare three types of such color lines comprised in the image sensor, theorder of adjacent color lines being such that one color line of thefirst type is followed by one of the second type and one of the secondtype is followed by one of the third type and one of the third type isfollowed by one of the first type.

Thus, three images are effectively detected by the image sensorsequentially, each time one of the entirety of color lines of one type,which represent the intensity of three spectral components of the samecolored picture. From the intensity of the spectral components in everyimage point, basically a colored image may be calculated.

However, a problem is posed by the fact that these three images do notreally represent the spectral components of the same image points sincethe color lines are offset from each other. If now these three imagesare taken for color evaluation, a systematic error occurs due to thefinite width of the color lines.

Therefore, the present invention is to solve the problem of providing anoptical measuring device of the above-mentioned kind, which avoids thissystematic error. This problem is solved by an optical measuring deviceof the above kind in that the illumination device comprises lightsources of different colors or other means that serve to alternativelyilluminate the measuring plane with different colors and in that theelectronic evaluation means detects the coloring of the test fields fromthe images obtained under different illumination.

Thus, the measuring device of the present invention does not require animage sensor having color lines but an image sensor whoselight-sensitive elements detect the intensity of the light receivedindependent from its wavelength. In the images obtained under differentcolor illumination, the same image point (i.e. the signal received underdifferent color illumination at the same sensor element) is generated bya light that has been reflected on the same point on the test strip.From the relative reflectivity of these points under different colorillumination, the respective color may be calculated without anysystematic errors occurring.

In a preferred embodiment, the image sensor comprises light-sensitiveCMOS components arranged in a planar matrix.

Preferably, the illumination unit uses colored LEDs as light sources. Ina preferred embodiment, the light sources comprise blue, green andorange LEDs, in particular LEDs having wavelengths of 450 nm, 530 nm and620 nm.

In order to uniformly illuminate a test strip in a measuring plane, itis advantageous to arrange the light sources in a series on atransmitter board. It is especially advantageous if the arrangementdensity of light sources on the transmitter board increases within theseries from the center in an outward direction. Due to suchinhomogeneous distribution of light sources on the transmitter board,the test strip may be almost homogeneously illuminated in the measuringplane.

In a further preferred embodiment, on both sides of the series of lightsources in parallel to the longitudinal direction of the series, screensare employed which focus the light emitted by the light sources on astrip-shaped portion in the measuring plane in which the test strip isto be placed. The screens preferably extend substantially over thelength of the series of light sources and comprise a plurality ofsurface segments extending over the entire length of the transmitterboard, wherein said surface segments' inclination angle relative to thetransmitter board increases in correspondence with an increasingdistance therefrom. With these screens, a test strip in the measuringplane may be illuminated with high intensity but nevertheless almosthomogeneously.

The screens preferably consist of parts that have been milled ormanufactured by injection molding, said parts having a reflecting layeror being laminated with a reflecting film. Thus, the screens are stableand can be manufactured at low cost.

As has been mentioned above, the color of a test field is detected bythe relative reflectivity of the test field for light of three differentwavelengths. If, for example, a test field is illuminated three times ina row with lights of different colors that, however, have differentintensities, the relative reflectivity corresponds to the relation ofintensities that are detected by the image sensor in the image of thetest field. However, it is not indispensably necessary to permanentlyilluminate the test strip with light of the same intensity. What isimportant is that the intensity of illumination is known, thus, therelative reflectivity can be calculated from the measured intensities atthe image sensor. Further, it may occur that the intensity ofillumination changes in operation. If, e.g. colored LEDs are used forthe illumination means, these are subject to wear which diminishes theirperformance.

For detecting a change of illumination intensity during operation it isadvantageous to provide a first reference surface that is arranged suchthat it is illuminated by the illumination unit together with the teststrip and imaged by the optical system onto the image sensor. By meansof the image of this reference surface, the electronic evaluation unitmay determine any illumination changes, such as a change in intensity ofillumination with one of the colors as a whole or only within a portionof the measuring plane. In case such change is determined, it isnecessary to newly calibrate the measuring device.

For calibration of the measuring device, the local intensitydistribution on the measuring plane for all three colors has to beknown. Preferably, the optical measuring device therefore has a secondreference surface adjustable between a first position in which it takesthe position of a test strip during measuring and a second position inwhich it cannot be imaged by the optical system onto the image sensor.The optical measuring device may thus calibrate itself by an adjustmentof the second surface to the first position, an illuminationsubsequently with the different colors and by using the images providedby the evaluation unit as calibration standard. Such calibrationprocedure may for example routinely be carried out by the measuringdevice upon switching on the measuring device or after a predeterminedamount of measurements has been made.

In an advantageous embodiment, the second reference surface is formed bya surface of a strip-shaped plate whose first end is equipped with afirst arm at least almost perpendicular with respect to said secondreference surface and pivotable about an axis parallel with respect tothe reference surface. By adjusting the arm, the second referencesurface can be adjusted between its first and second positions.Preferably, a second arm is further mounted to the second end of thestrip-shaped plate, said arm being substantially parallel with the firstarm and pivotable about the same axis as the first arm. The first arm isdischarged by the second arm.

Preferably, a bar is pivotally supported with its first end by the firstarm for adjusting the second reference surface between its first and itssecond position. In a preferred embodiment, this bar is mounted with itssecond end to a first lever pivotable about a first lever axis andbiased into a first lever position by means of a biasing element, inwhich position the reference surface takes its second position and thelever is adjustable against the biasing force of the biasing element toa second lever position in which the reference surface takes its firstposition. Preferably, the first lever is actuated by means of aneccentric drive.

The device according to the invention may be used as part of a teststrip analysis apparatus in which the test strips usually are advancedtowards the measuring means by means of belts or the like. However, whenmeasured, the test strip has to be in a static condition. This is mostsimply obtained when the conveying surface of the transport means of thetest strip analysis apparatus lies in the measuring plane and the teststrip is shortly kept in its measuring position on the conveyingsurface. In an advantageous embodiment, the optical measuring devicetherefore comprises a means for holding and aligning a test strip in itsmeasuring position.

Preferably, these means are defined by two at least almost parallel pinsmovable along their longitudinal axis between a first position in whichthey protrude into said measuring plane and a second position in whichthey are entirely outside said measuring plane. In their first position,these pins protrude into the measuring plane so that a test stripconveyed in the measuring plane is caught at the pins and alignsrelative to these pins in a measuring position. Preferably, the pins arebiased into their second position and adjustable against the biasingforce to their first position by a lever element. Preferably, the leverelement is adjusted by the same eccentric drive as the first lever.

In a particularly preferred embodiment, the eccentric drive is formed bya rotating disc driven by a motor, on the surface of which a pin isarranged perpendicularly thereto such that upon rotation of the disc inits first rotational direction, said pin engagingly moves said firstlever into its second lever position and that upon rotation of the discin its second rotational direction, said pin engagingly moves said leverelement such that said lever element moves said pins into their firstposition.

Even though, the optical measuring device according to the inventionmakes use of an illumination means that is able to alternativelyilluminate the measuring plane with different colors, all of thefeatures concerning the arrangement of the light sources, the screens,the first and second reference surfaces, the adjustment mechanism of thesecond reference surface and the means for holding and aligning a teststrip in its measuring position may also be advantageously used withconventional measuring means, in which white light for illuminating andfilters, i.e. image sensors having color lines are used.

Further advantages and features of the solution according to theinvention will become apparent from the following description whichoutlines the invention by means of an embodiment with reference to theaccompanying drawings, wherein

FIG. 1 is a perspective view of the optical measuring means according tothe invention;

FIG. 2 is a cross-section through a portion of the optical measuringdevice along A-A′ in FIG. 1,

FIG. 3 is a perspective view of the section of FIG. 2 with the secondreference surface in its first position,

FIG. 4 corresponds with FIG. 3, wherein the second reference surface isin its second position and a test strip in its measuring position, and

FIG. 5 is a perspective view of the illumination unit.

FIG. 1 illustrates a perspective view of the optical measuring device 10according to the invention, in which a test strip 12 extends in themeasuring plane 14 (cf. FIG. 2) in its measuring position.

FIG. 2 shows a cross-section through the measuring device of FIG. 1along A-A′. An illumination means 16 is illustrated by means of which astrip-shaped portion of the measuring plane, in which the test strip 12is in its measuring position, may be illuminated with different colors.FIG. 5 shows a perspective view of the illumination means 16. Theillumination means 16 comprise blue 18, green 20 and orange 22 LEDshaving wavelengths of 450 nm, 530 nm and 620 nm in a serial order on atransmitter board 24. This serial arrangement of the LEDs isparticularly suitable for illuminating the strip-shaped portion on themeasuring plane 14. Within the series, blue 18, green 20 and orange LEDs22 are respectively placed closely adjacent to one another in one groupso that the measuring plane is uniformly illuminated in all three colorswith the same intensity distribution. The groups of LEDs are arranged ina series such that their arrangement density of light sources on thetransmitter board increases within the series from the center in anoutward direction. This inhomogeneous arrangement of LEDs in a seriesresults in an almost homogeneous illumination intensity on the measuringplane in the field of the test strip.

In parallel to the longitudinal direction of the LED series, screens 26are disposed which focus the light emitted by the LEDs onto astrip-shaped portion, in which the test strip extends in its measuringposition. The screens 26 extend over the entire length of the LEDseries. They comprise four surface segments 28, 30, 32 and 34, extendingover the entire length of the screen 26. The inclination angle of thesurface segments increases relative to the transmission board 24 incorrespondence with an increasing distance therefrom. The screens 26 aremade of parts having been manufactured by injection molding and whosesurfaces 28, 30, 32 and 34 are provided with a reflecting layer.

The light emitted by the illumination unit 16 is diffusely reflected atthe test strip 12 and the test strip is imaged via an optical systemonto a planar image sensor 36, which is disposed in a housing 38 that isimpermeable against light. The optical system comprises a mirror 40, alens 42 and a screen 44. In FIG. 2 it is not the test strip 12 but thesecond reference surface 58 that is imaged onto the image sensor 36.Since the second reference surface 58, as will be described in moredetail below, takes the position of a test strip during measuring, theposition of the image 46 of the test strip is the same as the one of theimage of the reference surface. In FIG. 2, the generation of the image46 at the image sensor 36 is illustrated by means of two exemplary raysof light 48, 50. The optical path is folded at the mirror 40 thusmaintaining the measuring device in its entirety compact.

The image sensor 36 comprises a plurality of light sensitive CMOS partsgenerating a signal depending on the intensity of the light illuminatingthem. The light sensitive elements are arranged in a planar matrix andeach of them serves to generate an image point (pixel) of the image 46.

In the following, it shall be described how to detect the coloring ofthe test fields of the test strip 12 by means of the measuring deviceaccording to the invention. For this purpose, one of a plurality oflight sensitive sensor elements is selected. Light is imaged onto thesensor element by the optical system, the light being diffuselyreflected from a certain point, i.e. a very small portion, in one of thetest fields. Thus, the sensor element detects the intensity of the lightreflected at that point. If now this point is illuminated three timessequentially with blue, green and orange light having the sameintensity, the relations of intensities measured at the sensor elementrepresent the relative reflectivity for light of these three colors.Thus, however, the color of the test field is clearly determined. Thesignals generated at the image sensor 36 are transmitted to anevaluation unit 52 that detects the color of the original image, i.e. ofthe point or small portion on the test field, from these three signalsgenerated upon a three-time illumination at each of the image points.Due to the plurality of light sensitive sensor elements which arecomprised in the image sensor 36, the determination of color may beperformed sequentially for a corresponding number of pixels in the testplane.

For determining the relative reflectivity it is certainly not necessaryto illuminate each pixel in each color with light having the sameintensity. What is important is that the evaluation unit 52 gainsinformation about the relation of intensities of the different colors atone pixel in the measuring plane so that it may be taken into accountwhen calculating the relative reflectivity. By resorting to thisinformation, the measuring device may be calibrated according to theillumination conditions. However, the illumination of the measuringplane may vary in the course of time for a great number of reasons, e.g.due to the wear of the LEDs, defective LEDs or even only due tocontaminations in the illumination means. As a consequence, themeasuring device has to be calibrated anew.

For immediately determining during operation a change of intensity ofillumination of the measuring plane, the measuring device 10 accordingto the invention is provided with a first reference surface 54, which isarranged such that it is illuminated by the illumination unit 16together with the test strip and imaged by the optical system onto theimage sensor 36. This first reference surface 54 is illustrated in FIG.2 and its image on the image sensor 36 has been designated 56. As soonas there is a change of illumination, the image 56 changes on the imagesensor 36, which in turn is detected by the evaluation unit 52. Then,the evaluation unit 52 causes the self-calibration of the measuringdevice.

However, for calibrating the measuring means, the image 56 of the firstreference surface 54 cannot be resorted to since the illuminationintensity distribution on the first reference surface 54 is notidentical with the one in the measuring plane. Therefore, a secondreference surface 58 is provided which is adjustable from a secondposition, in which it is not illuminated by the illumination means 16and not imaged by the optical system onto the image sensor 36, to afirst position, in which it takes the position of a test strip duringmeasurement. The image of this second reference surface 58 serves tocalibrate the measuring means.

As shown in FIG. 1 and FIG. 4, the measuring device according to theinvention may be used for a test strip analysis apparatus, in which thetest strips are advanced towards the measuring device placed acrosstransport belts 60. It is particularly advantageous to provide aconstruction in which the transport belts 60 form a conveying surfacewhich coincides with the 35 measuring plane. Thus, the test strip 12 maybe measured lying on the transport belt 60. However, this requiresholding and aligning the test strip in its measuring position.

With regard to the illustrated measuring device two pins 62 are providedthat are movable along their longitudinal axis between a first positionin which they protrude into said measuring plane (cf. FIG. 1 and FIG. 4)and a second position in which they are entirely outside said measuringplane (cf. FIG. 2 and FIG. 3). The pins 62 are biased into their secondposition by a spring and adjustable to their first position by a leverelement 66 against the biasing force.

In the following, the adjustment mechanism of the pins 62 and the secondreference surface 58 is described. The second reference surface 58 isdefined by a surface of a strip-shaped plate 68 whose first end isequipped with a first arm 70 perpendicular relative to the referencesurface 58, this arm being pivotable with respect to an axis 72 (cf.FIG. 2) parallel to reference surface 58. A second arm is mounted to thesecond end of the strip-shaped plate, this arm being parallel withrespect to the first arm and also pivotable about the axis 72. This armis not shown in the drawings since it extends in a region of themeasuring device, that is cut away in FIGS. 2 to 4. The strip-shapedplate 68 and the first and second arms form a U-shaped part.

For adjustment of the second reference surface between its first and itssecond position, a bar 74 is pivotally mounted with its first end via ajoint 76 to a free end of the first arm 70. This bar is also pivotallymounted with its second end via a joint 76 to a first lever 78. Thisfirst lever 78 is pivotable about an axis 80 between a first and asecond position. This axis 80 is realized by a bolt, which, however, isnot shown in FIG. 3 and 4. In FIG. 4, the first lever 78 is in its firstlever position. The lever is biased into this position by means of aspring 82. Upon the lever 78 taking this first lever position, thesecond reference surface 68 is adjusted via the bar 74 and the first arm70 into its second position.

The first lever 78 is adjustable from its first into its second positionby means of an eccentric drive against the biasing force of the spring82. The eccentric drive is formed by a rotating disc 84 that is drivenby a motor, not shown, and on which a pin 68 is mounted perpendicularlythereto. Upon rotation of the disc in its first rotational direction(cf. FIG. 1 to 4, in a clockwise direction) the pin 86 engages the firstlever 78 and moves it into the second lever position, as shown in FIG. 2and 3, against the biasing force of the spring 82. Thus, the secondreference surface 68 is moved into its second position by means of thebar 74 and the first arm 70.

The lever element 66 is pivotable about an axis 88. This axis 88 isrealized by a bolt which is not shown in FIG. 3 and 4. The spring 82 isfixed to this bolt. In a position of the disc 84, in which the pin 86holds the second lever 78 in its second lever position against thebiasing force of the spring 82, as is the case in FIG. 2 and 3, the end90 of the lever element 66 is free so that the springs 64 of the pins 62relax and take their second position in which they lie completelyoutside the measuring plane. However, upon rotation of the disc 84 fromthis position in its second rotational direction (cf. FIG. 1 to 4, in aclockwise direction), the first lever 78 is moved by the spring 82 intoits first position, whereby the reference surface 68 moves into itssecond position. On the other hand, the pin 86 engages the end 90 of thelever element 66 after rotation about a certain angle and pivots thelever element about the axis 88 such that the pins 62 are moved intotheir second position against the biasing force of the spring 64. Thisis the position taken during measurement of a test strip, namely inwhich the second reference surface 68 is turned out of the optical pathand the pins 62 protrude into the measuring plane, so that a test strip12 may be aligned and held by them.

Therefore, by using a single eccentric drive, the adjustment mechanismdescribed facilitates adjustment between a calibration position, inwhich the second reference surface 68 takes its first position and inwhich the pins 62 take their second position, and a measuring position,in which the second reference surface 68 takes its second position andthe pins 62 take their first position.

1. An optical measuring device for detecting the coloring of test fieldsof a test strip to be wetted with a liquid for detecting substances insaid liquid, whereupon the reflectivity of the test fields changesdepending on the concentrations of the substances to be detected,comprising: a measuring plane upon which said test strip is to beplaced; an illumination device for illuminating the measuring plane andthe test strip; a planar image sensor; an optical system for providingan image of the test strip onto said planar image sensor; and anelectronic evaluation unit for evaluating at least one signal generatedby said planar image sensor, wherein said illumination device comprisesat least two light sources of different colors to alternativelyilluminate said test strip in said different colors such that saidelectronic evaluation unit detects the coloring of said test fieldsunder different color illuminations.
 2. An optical measuring deviceaccording to claim 1, wherein said image sensor comprises lightsensitive CMOS elements arranged in a planar matrix.
 3. An opticalmeasuring device according to claim 1 or 2, wherein said light sourcesof said illumination device comprise light emitting diodes (LEDs).
 4. Anoptical measuring device according to claim 3, wherein said LEDscomprise blue, green and orange LEDs having approximate wavelengths of450 nm, 530 nm and 620 nm, respectively.
 5. An optical measuring deviceaccording to claim 1, wherein said light sources are arranged in aseries on a transmitter board.
 6. An optical measuring device accordingto claim 5, wherein the arrangement density of said light sources on thetransmitter board increases from near the center of the transmitterboard in an outward direction towards the ends of the transmitter board.7. An optical measuring device according to claims 5 or 6, furthercomprising a screen on each side of the series of light sources, whereinsaid screens are disposed in parallel to the longitudinal direction of atest strip so as to focus light reflected from the light sources ontothe test strip.
 8. An optical measuring device according to claim 7,wherein said screens substantially extend over the length of the seriesof light sources and comprise a plurality of surface segments extendingover the entire length, wherein said surface segments' inclination anglerelative to said transmitter board increases in correspondence with anincreasing distance from the transmitter board.
 9. An optical measuringdevice according to claims 7 or 8, wherein said screens comprise partshaving been milled or manufactured by injection molding, said partshaving a reflecting layer or being laminated with a reflecting film. 10.An optical measuring device according to claim 1, further comprising afirst reference surface disposed such that it is illuminated by saidillumination device simultaneously with said test strip and imaged bysaid optical system onto said image sensor.
 11. An optical measuringdevice according to claim 1, wherein said measuring device comprises asecond reference surface that is movable between a first position inwhich it takes on the position of a test strip during measurement and asecond position in which it cannot be imaged by said optical system ontosaid image sensor.
 12. An optical measuring device according to claim11, wherein said second reference surface comprises a surface of astrip-shaped plate whose first end is equipped with a first armsubstantially perpendicular with respect to said second referencesurface, said first arm being pivotable about an axis parallel withrespect to said second reference surface.
 13. An optical measuringdevice according to claim 12, further comprising a second arm disposedat the end of the strip-shaped plate, said second arm beingsubstantially parallel with respect to the first arm and being pivotableabout the same axis as the first arm.
 14. An optical measuring deviceaccording to claims 12 or 13, further comprising a bar pivotallysupported by the first arm at the bar's first end for moving thereference surface from its first to its second position.
 15. An opticalmeasuring device according to claim 14, wherein said bar is pivotallysecured at its second end to a first lever, said first lever beingpivotable about a first lever axis and being biased into a first leverposition by means of a biasing element such that said reference surfacetakes its second position and wherein said first lever is adjustable toa second lever position against the biasing force of said biasingelement such that said reference surface takes its second position. 16.An optical measuring device according to claim 15, wherein said firstlever is actuated by means of an eccentric drive.
 17. An opticalmeasuring device according to claim 1, further comprising means forholding and aligning a test strip in its measuring position.
 18. Anoptical measuring device according to claim 17, wherein said means forholding and aligning comprise two substantially parallel pins movablealong their longitudinal axis between a first position in which theyprotrude into said measuring plane and a second position in which theyare entirely outside said measuring plane.
 19. An optical measuringdevice according to claim 18, wherein said pins are biased into theirsecond position and being adjustable to their first position against thebiasing force by means of a lever element.
 20. An optical measuringdevice according to claim 19, wherein said lever element is adjusted bythe same eccentric drive as the first lever.
 21. An optical measuringdevice according to claim 20, wherein said eccentric drive comprises arotating disc driven by a motor, and a second pin arrangedperpendicularly on a surface of said rotating disc such that uponrotation of the disc in a first direction of rotation, said second pinmoves said first lever into its second lever position and upon rotationof the disc in a second direction of rotation, said pin moves said leverelement such that said lever element moves said substantially parallelpins into their first position.